The present invention relates to electromagnetic devices such as permanent magnet alternators and permanent magnet motors.
Alternators are electromechanical devices that convert mechanical energy to alternating current (AC) electrical energy. Alternators employ a rotor to provide a rotating magnetic field that interacts with stator windings (i.e., conductors wound in coils around an iron core) to cause AC voltage to be generated. The magnitude of the AC voltage generated within the stator windings is directly related to the speed of the rotor and the strength of the magnetic field generated by the rotor.
Generating the magnetic field in the rotor can be done in a number of ways. For example, in wound field synchronous alternators, the rotor may include coils (commonly referred to as field windings) that are energized by providing current to the field windings. The AC power generated by the wound field synchronous generator is controlled by selectively varying the magnitude of the current provided to the field windings, and therefore controlling the strength of the magnetic field generated by the rotor. In the alternative, field windings wrapped around the rotor may be replaced by permanent magnets in what is known as a permanent magnet generator (PMG) or permanent magnet alternator (PMA). These type of machines are generally more efficient and robust than the wound field synchronous machines. However, because the strength of the magnetic field generated by the permanent magnets is constant, control of the AC voltage generated by PMGs is dependent on controlling the speed of the rotor. This drawback makes PMGs impractical in a number of applications, or requires PMGs to include additional hardware such as shunt voltage regulators in order to reduce AC voltage generated at increased rotor speeds. Therefore, it would be beneficial if PMGs could regulate the AC voltage (also referred to as electromotive force (emf)) generated in the stator windings at increased rotor speeds.
Motors are electromechanical devices that convert electrical energy (typically an AC signal) to mechanical energy. Motors work in much the same way as generators, except the direction of power is reversed (i.e., electrical energy is converted to mechanical energy). Motors generate mechanical energy by applying an AC signal to the stator windings. In permanent magnet motors, the AC signal applied to the stator windings generates a rotating magnetic field that interacts with the magnetic field produced by the permanent magnets located on the rotor. The interaction between the magnetic fields results in torque or force being generated on the rotor, causing it to turn. The speed at which the rotor rotates is a function, in part, of the magnitude of the current through the stator windings and the frequency of the current through the stator windings.
The maximum obtainable speed of permanent magnet motors is limited in part by the back electromotive force (bemf) generated in the stator windings by the rotating permanent magnets. The bemf opposes the AC voltage signal applied to create the AC current in the stator windings. As the magnitude of the bemf approaches the magnitude of the AC voltage signal, the amount of AC current that can be provided to the stator windings is reduced. This prevents the permanent magnet motor from further increases in speed. It would be desirable to reduce or limit the bemf generated in the stator coils to allow the permanent magnet motor to achieve higher speeds.
Therefore, it would be beneficial in both motor applications and generator applications to regulate the emf (commonly referred to as the bemf in motor applications).